1. Field of the Invention
The present invention relates to an optical filter and a display apparatus having the optical filter, and more particularly to an optical filter that is disposed on an outer surface of a face panel of a display apparatus such as a cathode-ray tube.
2. Description of the Related Art
As regards a cathode-ray tube that is typical as a display apparatus, Japanese Patent No. 2801600, for instance, discloses an optical filter by which the ratio in transmittance at a specified wavelength (e.g. 450 nm, 530 nm, 550 nm or 630 nm) can be set within a predetermined range, thereby to improve the contrast.
In general terms, in a cathode-ray tube, the inside of a vacuum envelope is kept in a high-vacuum state, and a compression stress and a tensile stress occurs in the vacuum envelope. If a mechanical shock acts on the vacuum envelope, there is a danger of implosion. To cope with this, the face panel of the vacuum envelope is formed to have a greater thickness at the peripheral part than at the central part, thereby to secure a sufficient mechanical strength for the cathode-ray tube.
In recent years, there has been an increasing demand for the flattening of the face panel. In order to secure the mechanical strength of the face panel, a difference in thickness between the peripheral part and the central part has been made still greater. With this face panel, the transmittance of the central part is high and that of the peripheral part is considerably low. As the difference in transmittance increases, non-uniformity in luminance tends to become visible, leading to a degradation in display image. In the prior art as disclosed in the above-mentioned Japanese Patent No. 2801600, no adequate consideration is given to the difference in transmittance due to the difference in thickness of the face panel.
Jpn. Pat. Appln. KOKAI Publication No. 11-283530, for instance, proposes a gradation coating method that aims at correcting such a difference in transmittance. In this method, the outer surface of the face panel is coated with a color film having a high transmittance at a part corresponding to a thick part of the face panel and a low transmittance at a part corresponding to a thin part of the face panel.
With this method, however, it is difficult to obtain a sufficient contact characteristic. There is known a method using BCP as an index for evaluating an improvement in the contrast characteristic. BCP is expressed byBCP=ΔB/√{square root over ( )}ΔRfwhere ΔB is the ratio of a decrease in luminance of a cathode-ray tube to be evaluated, relative to the luminance of a cathode-ray tube serving as a reference, and ΔRf is the ratio of a decrease in external light reflectance of the cathode-ray tube to be evaluated, relative to the external light reflectance of the reference cathode-ray tube. The BCP corresponds to a contrast improvement ratio in a case of considering, as a reference, a cathode-ray tube that includes a neutral filter having no maximum absorptance at a specified wavelength, that is, having a substantially constant transmittance to visible-region light.
In the cathode-ray tube having the neutral filter layer, the BCP is about 1.0 regardless of the transmittance, and an improvement in contrast is inadequate. In a cathode-ray tube with a light-selective absorption filter layer that has a maximum absorptance at a specified wavelength, the BCP deteriorates as the transmittance increases. In other words, a degradation in contrast is conspicuous. In the case of the cathode-ray tube with such a light-selective absorption filter layer, if the transmittance excessively decreases, the BCP can be improved and the contrast enhanced. However, with the effect of a component included in the light-selective absorption filter layer, such as neodymium oxide, which has a principal absorption band at a long wavelength, the filter color of the light-selective absorption filter layer itself varies due to external light. In particular, under an incandescent lamp, the film color of the light-selective absorption filter layer becomes red. Consequently, if the gradation coating method is applied, a display image becomes locally reddish and the display quality may deteriorate.